1. Field of the Invention
This invention relates to integrated circuits, and in particular to charge transfer devices as applied to charge coupled linear imaging devices.
2. Description of the Prior Art
Charge coupled devices are widely known as one type of integrated electronic device, and are widely used as portions of commercially available linear imaging devices, area imaging devices, and memory devices. In typical linear imaging devices of the prior art, a series of photosites accumulate charge in response to ambient lighting conditions or other stimulus. The accumulated charge is transferred out of the photosites into wells of a charge transfer register where the accumulated charge may be transferred to a sense amplifier or other apparatus for detection or use. In most commercially available linear imaging devices, charge from the photosites is clocked into only every other well of a charge transfer register adjacent and parallel to the linearly arranged photosites. Charge cannot be placed in every charge transfer well adjacent the photosites because if any packet of charge is moved to an adjacent transfer well, that packet of charge becomes mixed with the charge already in the adjacent well and destroys the information content of both wells.
One known technique which permits the use of every well is to use ripple clocking in the charge transfer register. In using ripple clocking the charge packet in the first well is transferred to an adjacent empty well, followed by transferring the charge packet in the second well to the now empty first well, transferring the charge packet from the third well to the now empty second well, etc. Unfortunately, such ripple clocking is difficult to generate and control accurately, particularly for long registers, and requires a relatively long period of time in which to empty a large linear charge transfer register. For example, if the charge transfer device is 5000 wells long and only the first well is empty, then 5000 clock cycles are required to advance all of the charge stored from each well to the adjacent well. Even if additional empty wells are periodically distributed throughout the transfer register, a large number of clock cycles is still required. Obviously, in high-speed applications such slow clocking is undesirable.
Accordingly, in typical linear imaging applications, the charge from photosites has been transferred only into every other charge transfer well. Thus an empty well separates every pair of wells in which charge is stored. Using only two clock cycles (in a two phase charge coupled device) the charge in every well is advanced one well to the adjacent empty well. Because an empty well always separates each well used to store a charge packet, the contents of the wells do not mix and the integrity of the information represented by the charge packets is maintained. Accordingly, in a linear imaging device 5000 units long, 5000 clock cycles will empty every well in the device.
An inherent disadvantage of the prior art device utilizing only every other charge transfer well is the decrease in resolution of the linear imaging device. For example, if the wells in the charge transfer register can be fabricated at best only 13 microns apart, the photosites from which charge is transferred to a given register of charge transfer wells must be 26 microns apart. Because the photosites from which charge is transferred are 26 microns apart, the resolution of the device is reduced. No prior art solutions are known which eliminate this disadvantage and permit doubling the resolution of the linear imaging device while operating it at the same speed.
One prior art structure, however, which utilizes a serpentine arrangement of charge transfer wells in conjunction with a linear imaging sensor is disclosed in "A Meander Channel CCD Linear Image Sensor" by H. Sei et al., IEEE Journal of Solid State Circuits, Vol. SC-13, No. 1, February, 1978, pages 66-70. In that journal article, a structure is described which is depicted schematically in FIG. 5 herein. This structure does not permit any substantial increase in the number of charge transfer wells per unit length of the charge coupled device.